The present invention claims the benefit of Korean Patent Application No. 2001-67193, filed in Korea on Oct. 30, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to a backlight device for use in the LCD device.
2. Discussion of the Related Art
In general, since flat panel display devices are thin, low weight, and have low power consumption, they are increasingly being used for displays of portable devices. Among the various types of flat panel display devices, liquid crystal display (LCD) devices are widely used for laptop computers and desktop monitors because of their superiority in resolution, color image display, and display quality.
LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce a predetermined image. Liquid crystal molecules have a definite orientation that results from their peculiar characteristics. The specific orientation can be modified by an electric field that is applied across the liquid crystal molecules. In other words, electric fields applied across the liquid crystal molecules can change the orientation of the liquid crystal molecules. Due to optical anisotropy, incident light is refracted according to the orientation of the liquid crystal molecules.
Specifically, the LCD devices have upper and lower substrates with electrodes that are spaced apart and face each other, and a liquid crystal material is interposed therebetween. Accordingly, when a voltage is applied to the liquid crystal material by the electrodes of each substrate, an alignment direction of the liquid crystal molecules is changed in accordance with the applied voltage to display images. By controlling the applied voltage, the LCD device provides various transmittances for rays of light to display image data.
The LCD device, however, does not emit the light by itself, and it only controls the light transmissivity. Therefore, the LCD device needs a light source additionally. As a light source, LCD devices often use backlight devices behind the LCD panel. The backlight devices are classified into direct back light type (or direct type) units and edge light type (or edge type) units, according to a position of a lamp. In the case where the direct type back light unit is used for a liquid crystal display device, incident rays irradiating from the lamp are directly incident to the LCD panel. In the case where the edge type back light unit is used, rays from the lamp are incident to the LCD panel via a light guide or a reflector. A detailed explanation of the edge type backlight will be provided subsequently.
FIG. 1 shows a conventional edge type backlight device for use in a LCD device. The conventional edge type backlight device includes a lamp 11, a U-shaped lamp housing 12 surrounding the lamp 11, a light guide plate 13, and a reflector 14. The light guide plate 13 coverts the light from the lamp into surface light and includes dot patterns (not shown) on the bottom thereof. The dot patterns (not shown) are formed, e.g., by printing of paint. The light guide plate 13 including the dot patterns diffuses light from the lamp in order to form the uniform surface light. The reflector 14 is disposed in the back side of the light guide plate 13, and serves to reflect the rays from the light such that light leakage is prevented. A first diffusion sheet 15, first and second prism sheets 16 and 17, and a second diffusion sheet 18 are sequentially formed on the front surface of the light guide plate 13.
FIG. 2 is a schematic perspective illustration of the first and second prism sheets 16 and 17 of FIG. 1. Each of the prism sheets 16 and 17 includes a plurality of triangular prisms 16a and 17a on the front surface thereof. The first triangular prisms 16a of the first prism sheet 16 is arranged perpendicular to the second triangular prisms 17a of the second prism sheet 17.
When the light guide plate 13 (in FIG. 1) includes the dot patterns printed by paint (i.e., a printing-type light guide plate), the backlight device preferably has more than two prism sheets which include perpendicular triangular prisms, respectively. However, as the number of prism sheets increases, total internal reflection frequently occurs in the angular prisms, thereby causing the optical transmission losses. Since the printing-type light guide plate utilizes light diffusion, it is very difficult to control the direction of the rays. Accordingly, prismatic patterns are widely used, on the bottom of the light guide plate instead of the dot patterns, as shown in FIG. 3. The prismatic patterns are formed by a non-printing method, such as an etching method.
FIG. 3 shows a conventional backlight device having a plurality of prismatic patterns. As shown, the conventional backlight device includes a lamp 21, a U-shaped lamp housing 22 surrounding the lamp 21, a light guide plate 23, and a reflector 24. The light guide plate 23 is thinner, further from the lamp 21. The light guide plate 23 includes a plurality of prismatic patterns 23a on the bottom thereof, which is formed by etching. The reflector 24 is disposed in the back side of the light guide plate 23, and serves as reflecting the rays from the lamp 21 such that light leakage is prevented. A diffusion sheet 25 and a prism sheet 26 are sequentially formed on the front surface of the light guide plate 23.
FIG. 4 is a simulation result showing viewing angle-dependence of luminance in cases of using the conventional backlight device of FIG. 3. The dotted line of the graph in FIG. 4 is a guide line that is expressed by cosine function. The guide line represents possible viewing angle distribution of the outgoing light from the backlight device. The line comprising an alternate sequence of long and short dashes indicates an up-and-down viewing angle distribution of the outgoing light from the backlight device of FIG. 3, while the full line indicates a right-and-left viewing angle distribution. As shown, the outgoing light from the backlight device distributes ranging from xe2x88x9230 to +30 degrees. When the viewing angle is zero, the intensity of the outgoing light is maximized.
Accordingly, since the backlight device having the prismatic patterns 23a of FIG. 3 can control the direction of the light from the lamp 21 using the light refraction, only one prism sheet is applied. As a result, the costs of production are reduced.
Meanwhile, the LCD device generally includes an absorptive color filter layer to display color images. However, when the light passes through the absorptive color filter, portions of the light are absorbed by the absorptive color filter. Thus, the optical loses increases and the luminance of the LCD device decreases. To overcome this problem, cholesteric liquid crystal (CLC) has been researched and developed for use as a color filter. The CLC color filter utilizes the selective reflection characteristics of the cholesteric liquid crystal. The LCD devices having the CLC color filter have great color reproduction and contrast ratio compared to those LCD devices that use an absorptive color filter.
As widely known, the cholesteric liquid crystal (CLC) reflects the light having a certain wavelength in accordance with its helical pitch, i.e., selective reflection characteristics. However, the light travels different distances with respect to the CLC color filter depending on different angles of incidence. Thus, the light passing through the CLC color filter experiences different helical pitches whenever it strikes the CLC color filter at different angles of incidence. As a result, the LCD device produces color shift in accordance with the viewing angles.
Accordingly, the present invention is directed to a backlight device for a liquid crystal display (LCD) device, which substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a backlight device for a liquid crystal display device, which has a high light collimating efficiency.
Another advantage of the present invention is to provide a backlight device for a liquid crystal display device, which is produced at relatively low costs.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a backlight device for a liquid crystal display device includes at least a lamp; at least a lamp housing surrounding the lamp and having an opening on a side; at least a light guide plate connected to the opening of the lamp housing, the light guide plate guiding light from the lamp; a reflector arranged on the rear surface of the light guide plate, the reflector reflecting the light from the lamp in a direction to the light guide plate; a pre-collimator disposed in the lamp housing between the lamp and the light guide plate; a diffusion sheet disposed on the front surface of the light guide plate; and a prism sheet disposed on the diffusion sheet; wherein the lamp housing has lower and upper portions; and wherein the lower and upper portions have first and second V-shaped grooves in the inner surfaces thereof. The first and second V-shaped grooves correspond and face to each other, so the pre-collimator is fixed into the V-shaped grooves.
In another aspect, a backlight device for a liquid crystal display device includes at least a lamp; at least a lamp housing surrounding the lamp and having an opening on a side; at least a light guide plate connected to the opening of the lamp housing, the light guide plate guiding light from the lamp; a reflector arranged on the rear surface of the light guide plate, the reflector reflecting the light from the lamp in a direction to the light guide plate; a pre-collimator disposed in the lamp housing between the lamp and the light guide plate; a diffusion sheet disposed on the front surface of the light guide plate; and a prism sheet disposed on the diffusion sheet; wherein the lamp housing has lower and upper portions; and wherein the lower portion has a V-shaped groove in the inner surfaces thereof and the upper portion has a hole covered by a hole cover. The V-shaped groove of the lower portion corresponds and faces to the hole of the upper portion, and thus, the pre-collimator is inserted through the hole of the upper portion and fixed into the V-shaped groove of the lower portion. The hole cover is a silver (Ag) coated tape.
In another aspect, a backlight device for a liquid crystal display device includes at least a lamp; at least a lamp housing surrounding the lamp and having an opening on a side; at least a light guide plate connected to the opening of the lamp housing, the light guide plate guiding light from the lamp; a reflector arranged on the rear surface of the light guide plate, the reflector reflecting the light from the lamp in a direction to the light guide plate; a pre-collimator disposed in the lamp housing between the lamp and the light guide plate; a diffusion sheet disposed on the front surface of the light guide plate; and a prism sheet disposed on the diffusion sheet; wherein the lamp housing has lower and upper portions; and wherein the lower and upper portions have Z-bands each having a step. The Z-bends allows the lamp housing to have inner and outer widths between the lower and upper portions. The outer width is larger than the inner width by 400 to 600 micrometers. The pre-collimator is disposed at the Z-bends of the upper and lower portions of the lamp housing. The light guide plate and the reflector are attached to the pre-collimator to fix the pre-collimator. The step of Z-bends ranges from 200 to 300 micrometers.
With respect to the above-mentioned backlight device, the lamp housing has a U shape. The pre-collimator is formed of prism sheet that has a plurality of angular prism. The light guide plate includes a plurality of prismatic patterns at the bottom.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.